Process for compounding rubber



United States Patent 3,324,075 PROCESS FOR COMPOUNDING RUBBER Nathan Burak, Prestwich, near Manchester, England, as-

signor to Philadelphia Quartz Company, Philadelphia, Pa., a corporation of Pennsylvania No Drawing. Filed Oct. 7, 1964, Ser. No. 402,325

' 19 Claims. (Cl. 260-415) This invention relates to reinforced rubber, more particularly reinforced rubber in which the reinforcing agent consists of or comprises a silica or silicate.

The incorporation of a reinforcing silica or silicate in rubber mixes is accompanied by an increase in the viscosity of the mix; this is particularly pronounced at high loadings of the reinforcing agent.

It is an object of the invention to provide an improvement inthe processingof rubber with a reinforcing silica or silicate.

According to the invention there is provided a process of compounding a rubber mix containing a reinforcing silica or silicate in which process the viscosity of the mix is reduced by the inclusion therein of an organic silicate ester.

The rubber in the rubber mix employed in the process provided by this invention can be wholly natural rubber, wholly synthetic rubber or a blend of natural and synthetic rubbers of any desired ratio. Typical synthetic rubbers useful in this invention include polyisoprene rubber, butyl rubber, styrene butadiene rubber and polybutadieue rubber.

It has been found that a substantial reduction in the viscosity of the rubber mix can be obtained by the inclusion of a relatively small amount of an organic silicate ester. The extent of the reduction in viscosity is dependent on the amount of the ester added. While even with amounts of ester of less than 2% based on the weight of the silica or silicate, a significant decrease in viscosity may be obtained it will usually be necessary to use from 2% to 25% to get the best results. Amounts above 25% by weight based on the silica or silicate can also be used but at the higher levels of addition the further reduction in viscosity tends to be relatively small and with such additions the mechanical properties of thecured rubber may be undesirably affected. The .preferred amount of ester is in the range 4 to 20% by weight based on the weight of the silica or silicate reinforcing agent. Based on the total weight of the rubber mix, the weight of the silicate ester used will normally be within the range 0.5 to usually from 1 to 7%.

The organic silicate may be an aliphatic, phenylsubstituted aliphatic, aromatic, aliphatic-substituted aromatic, or a mixed aliphatic aromatic silicate ester. As a further example of a suitable class of esters may be mentioned the mixed al-kyl acyl silicate esters.

Examples of aliphatic silicate esters which may be used in the process of the invention are those in which the aliphatic group is an alkyl or alkenyl group having up to carbon atoms. Typical of such esters are the ethyl, isopropyl, allyl, sec-amyl, and n-hexyl silicates.

As examples of suitable phenyl-substituted aliphatic silicate esters are phenyl-substituted alkyl silicate esters in which the alkl group contains up to 4 carbon atoms, such as the benzyl silicates.

The pheny silicates are typical examples of aromatic silicate esterswhich may be employed.

As exemplifying the class of aliphatic-substituted aromatic silicate esters are mentioned the alkaryl silicate esters in which the alkyl substitutent in the .phenyl nucleus has up to 18 carbon atoms, for example the dodecylphenyl silicates.

Of the mixed aliphatic aromatic silicate esters, that is silicate esters containing both aliphatic and artomatic 3,324,075 Patented June 6, 1967 ester groups, examples are the mixed alkyl aryl silicate esters, the alkyl group or groups preferably containing up to 6 carbon atoms. As specific members of this class of esters may be mentioned the isopropyl phenyl silicates. In this class of esters there is preferably at least 0.5 aliphatic ester groups per aromatic ester group.

As examples of suitable mixed alkyl acyl silicate esters may be mentioned those in which the alkyl group contains up to 4 carbon atoms and the acyl group up to 18 carbon atoms; particular examples are the ethyl .palmitoyl silicates.

The silicate ester may be an orthosilicate or a polysilicate or a mixture of these silicates.

The silicate ester may be included in the rubber mix in any suitable manner. In the case of a liquid ester, it is conveniently added made up as a paste with a part of the silica or silicate reinforcing agent.

The reinforcing silica employed may be a precipitated silica or a fume silica (also known as a pyrogenic silica). Examples of suitable reinforcing silicates are the precipitiated calcium silicates, precipitated calcium aluminium silicates and the precipitated sodium aluminium silicates. Mixtures of a reinforcing silica and silicate may also be used.

The silica or silicate reinforcing agent may be used alone or in combination with other rubber fillers, for example c'arbon blacks or calcium carbonate. The silica or silicate can be employed with other rubber fillers over a wide range of proportions. Usually, for practical purposes, the weight of the siliceous material will be at least 25 of the total weight ofthe filler mixture.

'It has furthermore been found that when some esters are used, more especially when the reinforcing agent consists of or comprisessilica and when a natural rubber or a rubber mix containing a large proportion of natural rubber is used, as well as obtaining the advantage of easier compounding, the final cured product has an improved abrasion resistance. The improvement in abrasion resistance is most pronounced in natural rubber compositions but certain synthetic rubbersparticularly polyisoprene can be blended in significant proportions with natural rubber and the improvement is still detectable. The preferred silicates in this respect are the alkyl polysilicate's in which the alkyl group has up to 4 carbon atoms, particularly the ethyl and isopropyl polysilicates, and the hexyl and di-palymitoyl di-ethyl orthosilicates. In those instances where non-siliceous fillers are included in the rubber mix, the amount of silicate ester to be used will usually be greater than that required to effect a comparable increase in the abrasion resistance of a rubber based on an all-siliceous filler.

The invention will now be illustrated by the following examples. All parts are by weight.

Example 1 The formulation (Formulation 1) of the rubber compound tested is set out below.

Vulcanisation: 15 and 30 mins. at 142 C. 1 As indicated below.

The compound was milled in a conventional manner,

the silicate ester being added made up into a thick paste with a part of the silica.

The following properties were measured:

(a) The Mooney viscosity (ML 4) at 4 minutes at 120 C. and the Mooney scorch time at 120 C. according to B.S. 1673: Part 3:1951.

(b) The tensile strength, modulus at 300% elongation, percentage elongation at break, tear strength and hardness (B.S. degrees) of the cured compounds by the methods The invention will now be ilustrated by the following examples. All parts are by weight.

Example 2 The formulation of the rubber compound is set out below.

specified in B.S. 903. Parts For comparison purposes, the properties mentioned Cls-polylsoprene rubber (synthetlc) 100 above were also measured on the formulation obtained by Reinforcing grade preclpltated silica 55 leaving out the silicate ester.

The results are set out in Table I.

TABLE I Tensile Modulus Elong. to Tear Strength, 300% Break, Strength, Hardness, p.s.i. Elong., Percent p.s.i. B.S. Mooney p.s.i. Silicate Ester Parts of Mooney Scorch ester Viscosity Time (mins.) Cure Time (minutes) Isopropyl polysilicate (37-42% 510;) 5 36 40 3, 600 3, 420 730 840 750 675 2, 670 2, 150 68 74 Isopropyl polysilicate 7 30 40 3, 540 3, 520 970 920 625 600 2, 250 2, 050 73 77 Sec-amyl polysilicate (29% S10 3 41 40 3, 095 3, 245 570 730 700 050 2, 100 2, 270 60 Ethyl polysilieate (40% S10 3 50. 5 40 3, 580 3, 525 795 915 700 650 2, 625 2, 330 71 Ethyl orthosilicate 5 48 40 3, 975 3, 700 900 925 700 650 2, 425 2, 000 78 n-Hexyl orthosilieate. 3 61 40 3, 660 3, 400 740 875 700 600 2, 750 2, 350 71 74 Dodecyl-phenyl polysilicate (55% S10 5 62 40 2, 045 3, 670 200 765 975 750 625 2, 550 60 73 Phenyl isopropyl polysilicate (1;1 ratio of phenyl to isopropyl groups; 33% S10 5 44. 5 40 2, 720 3, 920 580 1, 000 725 675 1, 750 2, 345 05 78 Dtpalmitoyl di-ethyl orthosilicate 5 47 40 3, 880 3, 540 840 840 750 700 2, 410 1, 810 77 80 Allyl orthosilieate 4 3 53. 5 40 2, 745 2, 900 465 005 700 700 1, 075 2, 005 66 68 Control (Formulation 1 without ester) (average of 30 tests) 83. 5 40 3, 533 3, 508 700 788 730 685 2, 550 2, 485 71 74 The above results show that the inclusion of a silicate 35 Zinc oxide 2.2

ester results in a considerable decrease in the viscosity of the mixture while at the same time the results show that, although the cure time in some instances may be retarded, the properties of the rubber compound referred to in the table are, except when using allyl orthosilicate, on the whole not adversely affected to any significantextent.

Table II below gives similar data to that presented in Table I for rubber compounds corresponding to Formulation 2 given below. In one case, the precipitated silicate was a sodium aluminium silicate and in another case a calcium silicate was employed. For both of these silicate materials comparative tests were done on the compound Sulphur Diethylene glycol 3.5

Hexamethylene tetramine 1 Vulcafor I-I.B.S. (N-cyclohexyl-2-be-nzothiazole sulphenamide) 1.6

Tetra-methyl thiuram disulphide 0.7

Isopropyl polysilicate Vulcanisation: 15 mins. at 142 C.

1 As indicated in the table.

obtained by omitting the isopropyl polysilicate from the The compound was milled in a conventional manner, formulation. 50 the silicate ester being added to make up into a thick Formulation Parts paste with a part of the silica.

Natural n'lbber The following properties were measured: Zinc oxide u 5 (a) The Mooney viscosity (ML 4) at 4 minutes at Stearic acid n 3 C. and the Mooney scorch time at 120 C. accord- Vulcafor HBS (N-cyclohexyl-2-benzothiazole 55 mg to 167%: Part 31951 sulphenamide) (b) The tenslle strength, modulus at 300% elongation, Precipitated Silicate 60 percentage elongation at break, tear strength and hard- Sulphur 3 ness (B.S. degrees) of the cured compounds by the meth- Non-staining antioxidant 1 ods specified in B.S. 903. Isopropyl polysilicate 3 60 Vulcanisation: 15 and 30 mins. at 142 C.

TABLE II Tensile Modulus Elong. to Tear Strength, 300% Break, Strength, Hardness, Mooney p.s.i. Elong., p.s.i. Percent p.s.i. B.S. Mooney Scorch Precipitated Silicate Viscosity Time (mins.) Cure Tune (minutes) Sodium Aluminum Silicate:

Silicate ester present 19.5 19 3,120 2,790 1,250 1,200 550 500 1,430 1,465 68 74 Silicate ester absent 43.5 18 2, 970 2,830 1, 520 1,575 500 500 1,570 1, 630 70 77 Calcium silicate:

Silicate ester present 11.5 20 2,700 2, 520 1,065 1,005 500 500 1,020 970 09 09 Silicate ester absent 13.0 19 2,710 2, 430 1,005 900 500 500 1,160 1,100 07 07 The results are set out in the table.

Example 3 The formulation of the rubber compound of this ex- 6 Example 7 The formulation of the rubber compound of this example is set out below.

ample is set out below. P t 5 Parts Stereo specific polybutadiene rubber (40% cis) 40 Buiyl ruijber '7'. T. u 100 Natural rubber 60 Rpmforiimg grade preclpltated slhca Reinforcing grade precipitated silica 50 Zinc Die-thylene glycol 2 Steanc acld 2 Zlnc oxide 1.5

Tetra-methyl thiu-ram disulphide 1.5 Stearic acid 1 5 lsi'lliriaptobenzthlazole 1 Hydrocarbon processing oil 10 P "r.- Vulcafor F (a blend of di-2-benzothiazyldisulphide {j f i l i gg l {'Eig'f and diphenyl guanidine) 1.25

H camsa mmu es a Tetra-methyl thiuram monosulphide 0.2

1As indicated in the table. Sulphur 2.25

The results are set out in the table. Is-opropyl polysicilate Vulcanisation: 15 minutes at 144 C. Example 4 The formulation of the rubber compound of this ex- 1A5 unheated in the ample i Set out b l The results are set out in the table.

, Parts Styrene butadiene rubber 100 Example 8 Reinforcing grade preciplmted Sihca 69 The formulation of the rubber compound or this ex- Z11: Oxide 5 ample is set out below. Stearic acid 3 Parts 1103501: F blend Pf dl-z-benzothlalyl dlsulphlde Stereo specific polybutadiene rubber (40% cis) 40 and 'dlphenyl Styrene butadiene rubber 60 HydroCarbon Processing 011 5 Reinforcing grade precipitated silica 5O sulphur Diethylene glycol 3 Isopropyl polysllicate (1) Zinc oxide L5 Vulcanisation: 30 minutes at 142 C. m; acid 1'5 1 As indicated in the table. Hydrocarbon processing oil 10 The results are Set out in the table Vulcaf-or F (a blend of d1-2-benzoth1azyld1sulph1de and diphenyl guanrdme) 1.75 Example 5 Tetra-methyl thiuram monosulphide 0.5

The formulation of the rubber compound of this exu phur 1.75 ample was the same as that of Example 2 save that 30 Is-opropyl polysllrcate parts of the isoprene rubber were replaced by 30 parts V-ulcamsatron: 15 minutes at 144 C of natural rubber. In this instance the cure time was 15 1 As indicated in the table minutes.

The results appear in the table. The results are set out in the table.

TABLE Parts of Mooney Mooney Tensile ll/Iodulus Elong. to v Tear Hardness Ex. Type of Rubber isopropyl Viscosity Scorch Strength at 300% Break, Strength, B.S.)

polysili- (ML 4) T me (p.s.1.) Elong. Percent (p.s.1.)

cate (rn1ns.) (ps.1.)

h 0 s2 12 2,565 610 800 1, 535 85 2 Isopreue (synt etie) I 3 76 11 2,635 635 800 1,465 81 Butyl 0 11s 25 1,900 240 900 1, 090 75 5 s4 27 1,695 270 1, 000 785 55 4 Styrene butadiene 0 152 40 2,130 455 950 1,345 85 3 102 40 2, 120 7,5 650 s 85 5 op Natural g g; 12 ggg 28g 253 g? 5 e5 26 31335 850 750 1; 020 so 6. 70% Isoprene, 30% Styrene butadiene a. g 3Z2 3, 1, 5 7e 21 1:835 685 500 555 7s 7 40% Stereo specific polybutadiene, 60% 0 82 17 1,920 460 700 955 74 5 57 20 2, 450 535 575 1, 480 72 Natural 7 45 20 2, 495 560 500 1, 230

8 40% Stereo specific polybutadiene, 0 63 12 1,840 710 600 695 73 Styrene butadiene. 5 49 17 1,505 640 600 585 71 Example 6 The formulation of the rubber compound of this example was the same as that of ExampleZ save that 30 It W111 f from the t table that every parts of the isoprene rubber were replaced by 30 parts stance the me usion of the s1l1cate ester has caused a lowof styrene butadiene rubber. In this instance the cure enng of me Mooney Y- time was 15 minutes. The isopropyl polysilicate employed in the above ex- The results are set out in the table. 75 amples had a silica content of 37-42% by weight.

7 Example 9 In this example the effect, on the abrasion resistance of the rubber compound of Formulation 1, of the inclusion of certain silicate esters is set out.

The method of determining abrasion resistance used is that described in B.S. 9031Part A 911957 using the Du Pont Abrasion Tester.

To check the relative effect of the organic silicates on the abrasion indices of silica rubber compounds, a series of tests was carried out using recommended carbon black formulations (see Formulation 3). The blacks employed were ISAF Black and HAF Black; these grades of carbon black are reputed to be of high abrasion resistance, ISAF being used in tyre treads and HAF in tyre walls. In addi tion, in the B.S. 903 Abrasion Index measurement, the silica loaded compound was always compared with either Formulation 4, which is an EPC Black and whiting compound termed in B.S. 903 Compound B, or with Formulation 5, which is an EPC Black compound termed in B.S. 903 Compound A.

The work with Formulations 4 and 5 provided a comparative scale against which the effects of the organic silicates on the rubber compound may be judged.

Formulation 3: Parts Smoked sheet 100 Zinc oxide 5 Stearic acid 3 Carbon black 50 Dutrex R (a hydrocarbon processing oil) 4 Sulphur 2.5 Vulcafor HBS (N-cyclohexyl-Z- benzothiazole sulphonamide) 0.5 Vulcanisation: 30 and 50 mins. at 135 C.

Formulation 4: Parts Smoked sheet 100 Zinc oxide 4 Stearic acid 3 Di-Z-ethyl hexyl phthalate 3 E.P.C. black 6O Whiting 60 Mercaptobenzthiazole 1 Sulphur 3 Phenyl-beta-naphthylamine l Vulcanisation: 40 minutes at 153 C.

Formulation 5: Parts Smoked sheet 100 Zinc oxide 5 Stearic acid 3 E.P.C. black 50 Benzthiazyl disulphide 1 Sulphur 3 Phenyl-beta-naphthylamine l Vulcanisation: 40 minutes at 144 C.

Formulation 6: Parts Smoked sheet 30 Cis-polyisoprene rubber 70 Zinc oxide 2.2

Stearic acid 2 Reinforcing grade precipitated silica 55 Non-staining antioxidant 0.8 Sulphur 2.1 Diethylene glycol 3.5 Hexamethylene tetramine 1 Vulcafor H.B.S. (N-cyclohexyl-2-benzothiazole sulphenamide) 1.6 Tetra-methyl thiuram disulphide 0.7 Isopropyl polysilicate Vulcanisation: mins. at 142 C.

1 As indicated,

The results are set out in Table III.

TABLE III Abrasion Re- Volume sistanee Index loss Compared to- Natural Rubber Formulation mlsi 1,0 revs Formu- Formulation 4 lation 5 Formulation 4 2. 42 100 67 Formulation 5 1. 63 148 100 Formulation 3 using HAF Black 1. 601 151 102 Formulation 3 using ISAF Black 1. 522 159 107 Formulation 1 comprising-- 1 part isopropyl polysilieate 2. 03 119 3 parts isopropyl polysilicate. 1. 504 161 108 5 parts isopropyl polysilicate. 1. 112 218 147 7 parts isopropyl polysilicate. l. 043 232 150 9 parts isopropyl polysilicate. 0.945 255 172 3 parts n-hexyl orthosilicate.. 1. 693 143 96 5 parts n-hexyl orthosilicate" 1. 541 157 106 3 parts ethyl polysilieate 1.917 126 5 parts phenyl isopropyl polysilicate (phenyl-isopropyl group ratio= 1:1 2. 2 74 5 parts di-palmitoyl di-ethyl orthosilicate 2. 03 119. 2 80. 3 Formulation 6 compri parts isopropyl polysilieate 1. 45 112. 4 160. 9

Silica Control (Formulation 1 without ester) (average of 5 sets, i.e. 30 tests) 2.38 101 68 The above results clearly demonstrate the improvement in abrasion resistance resulting from the incorporation of the silicate ester. In particular it will be noted that with isopropyl polysilicate and natural rubber, save for the smallest addition of silicate, the abrasion resistance is better than that of Formulation 3 using ISAF Black as reinforcing agent.

It is of interest to mention that in experiments on Formulation 3 using HAF Black, the inclusion of 3 and 5 parts of isopropyl polysilicate resulted in an increase in the viscosity of the rubber mix and a decrease in the abrasion resistance of the cured compound. The experiment with Formulation 6 indicates that with quite high proportions of at least one synthetic rubber in a mix containing natural and synthetic rubbers a significant increase in abrasion resistance can be obtained when an appropriate quantity of a suitable organic silicate ester is employed in an adequate quantity.

What is claimed is:

1. A process of compounding a rubber mix containing a reinforcing agent selected from the group consisting of silica and silicate, in which process the viscosity of the mix is reduced by the inclusion therein of from 2 to 25% by weight based on the weight of the said reinforcing agent of an organic silicate ester.

2. A process as claimed in claim 1, wherein the said reinforcing agent is a precipitated silica.

3. A process as claimed in claim 1 wherein the silicate ester is an aliphatic silicate ester in which the aliphatic group has up to 15 carbon atoms.

4. A process as claimed in claim 3 in which the ester is an alkyl silicate ester.

5. A process as claimed in claim 4 wherein the alkyl group has up to 4 carbon atoms.

6. A process as claimed in claim 5 wherein the ester is selected from the group consisting of ethyl and isopropyl silicate.

7. A process as claimed in claim 6 wherein the ester is a polysilicate.

8. A process as claimed in claim 1 wherein the ester is an orthosilicate.

9. A process as claimed in claim 8 wherein the ester is n-hexyl orthosilicate.

10. A process as claimed in claim 1 wherein the ester is a mixed alphatic aromatic silicate ester.

11. A process as claimed in claim 10, wherein the ester is a mixed alkyl phenyl silicate ester in which the alkyl group has up to 6 carbon atoms.

12. A process as claimed in claim 11 wherein the ester is an isopropyl phenyl silicate ester.

13. A process :as claimed in claim 11 wherein the ester is a polysilicate.

14. A process as claimed in claim 1 wherein the ester is a mixed alkyl acyl silicate.

15. A process as claimed in claim 14 wherein the ester is an ethyl palmitoyl orthosilicate.

16. A process as claimed in claim 1 wherein the said silica or silicate is used in conjunction with a further reinforcing agent.

17. A process as claimed in claim 1 in which the rubber in the mix is selected from the group consisting of natural rubber and predominantly natural rubber mixes.

18. A process as claimed in claim 1 in which the rubber in the mix comprises natural rubber and polyisoprene rubber.

References Cited FOREIGN PATENTS 536,033 3/1955 Belgium.

MORRIS LIEBMAN, Primary Examiner. S. L. FOX, Assistant Examiner. 

1. A PROCESS OF COMPOUNDING A RUBBER MIX CONTAINING A REINFORCING AGENT SELECTED FROM THE GROUP CONSISTING OF SILICA AND SILICATE, IN WHICH PROCESS THE VISCOSITY OF THE MIX IS REDUCED BY THE INCLUSION THEREIN OF FROM 2 TO 25% BY WEIGHT BASED ON THE WEIGHT OF THE SAID REINFORCING AGENT OF AN ORGANIC SILICATE ESTER. 